1. Technical Field
The present invention relates to a satellite signal receiving device that performs positioning and adjusts the time based on signals received from positioning information satellites such as GPS satellites, to a control method for the satellite signal receiving device, and to an electronic device.
2. Related Art
Electronic devices that receive satellite signals from GPS (Global Positioning System) satellites and perform positioning and time adjustment are known from the literature (for example, document 1, Japanese Unexamined Patent Appl. Pub. JP-A-2008-39565).
When the electronic device is a device, such as a wristwatch, that moves with the user, the electronic device could conceivably move to an environment where satellite signals cannot be received, such as indoors or an underground mall.
If the reception process is executed in such an environment where satellite signals cannot be received, power is wasted. Reducing current consumption and avoiding wasteful reception processes are particularly important in battery-powered electronic devices such as wristwatches to assure sufficient duration time and reduce the battery size.
As a result, document 1 describes providing a solar panel in the electronic device, determining if the electronic device is outdoors by comparing the power output with a threshold value for determining if the electronic device is indoors or outdoors, and performing the reception process if determined to be outdoors.
However, this is based on solar panel power output corresponding to the illuminance of light incident to the solar panel. It is also based on being able to make an indoor/outdoor determination by obtaining power output corresponding to the illuminance when the electronic device is outdoors during the day and the illuminance when indoors, and setting the threshold value so that power output can be differentiated in these cases.
In reality, however, depending upon where the electronic device is used, power output can exceed the threshold value even if the electronic device is indoors, and the reception process may therefore be executed in an environment where satellite signals cannot be received.
There are also situations in which satellite signals cannot be received when the electronic device is outdoors even if the reception process is executed, such as when in an urban canyon between tall buildings. The electronic device being outdoors therefore does not mean that the environment is necessarily suited to receiving satellite signals.
Therefore, if the threshold value is fixed as described in document 1, the reception process is often executed even in environments where satellite signals cannot actually be received, and power consumption can therefore increase as a result.
Furthermore, depending on the operating conditions of the electronic device, power output may also not exceed the threshold value even when the electronic device is outdoors. For example, when the electronic device having a satellite signal receiving device is a wristwatch, power output may not exceed the threshold value even though the electronic device is outdoors if the solar cell is covered by a sleeve, for example. Depending upon the season or the weather, power output may also not exceed the threshold value even though the electronic device is outdoors because direct sunlight is not incident or is weak. In addition, when the solar cell has been used for a long time and solar cell deterioration is advanced, power output may not exceed the threshold value even though light of the same illuminance is incident to the solar cell.
As a result, when the threshold value is fixed as described in document 1, there is also the possibility that the reception process will not be performed even in an environment that is actually suited to satellite signal reception.